1. Field of the Invention
Embodiments of the present invention generally relate to nickel-base alloys and methods of heat treating nickel-base alloys. More specifically, certain embodiments of the present invention relate to nickel-base alloys having a desired microstructure and having thermally stable mechanical properties (such as one or more of tensile strength, yield strength, elongation, stress-rupture life, and low notch sensitivity). Other embodiments of the present invention relate to methods of heat treating nickel-base alloys to develop a desired microstructure that can impart thermally stable mechanical properties at elevated temperatures, especially tensile strength, stress-rupture life, and low notch-sensitivity, to the alloys.
2. Description of Related Art
Alloy 718 is one of the most widely used nickel-base alloys, and is described generally in U.S. Pat. No. 3,046,108, the specification of which is specifically incorporated by reference herein.
The extensive use of Alloy 718 stems from several unique features of the alloy. For example, Alloy 718 has high strength and stress-rupture properties up to about 1200° F. Additionally, Alloy 718 has good processing characteristics, such as castability and hot-workability, as well as good weldability. These characteristics permit components made from Alloy 718 to be easily fabricated and, when necessary, repaired. As discussed below, Alloy 718's unique features stem from a precipitation-hardened microstructure that is predominantly strengthened by γ″-phase precipitates.
In precipitation-hardened, nickel-base alloys, there are two principal strengthening phases: γ′-phase (or “gamma prime”) precipitates and γ″-phase (or “gamma double prime”) precipitates. Both the γ′-phase and the γ″-phase are stoichiometric, nickel-rich intermetallic compounds. However, the γ′-phase typically comprises aluminum and titanium as the major alloying elements, i.e., Ni3(Al, Ti); while the γ″-phase contains primarily niobium, i.e., Ni3Nb. While both the γ′-phase and the γ″-phase form coherent precipitates in the face centered cubic austenite matrix, because there is a larger misfit strain energy associated with the γ″-phase precipitates (which have a body centered tetragonal crystal structure) than with the γ′-phase precipitates (which have a face centered cubic crystal structure), γ″-phase precipitates tend to be more efficient strengtheners than γ′-phase precipitates. That is, for the same precipitate volume fraction and particle size, nickel-base alloys strengthened by γ″-phase precipitates are generally stronger than nickel alloys that are strengthened primarily by γ′-phase precipitates.
However, one disadvantage to such a γ″-phase precipitate strengthened microstructure is that at temperatures higher than 1200° F., the γ″-phase is unstable and will transform into the more stable δ-phase (or “delta-phase”). While δ-phase precipitates have the same composition as γ″-phase precipitates (i.e., Ni3Nb), δ-phase precipitates have an orthorhombic crystal structure and are incoherent with the austenite matrix. Accordingly, the strengthening effect of δ-phase precipitates on the matrix is generally considered to be negligible. Therefore, as a result of this transformation, the mechanical properties of Alloy 718, such as stress-rupture life, deteriorate rapidly at temperatures above 1200° F. Therefore, the use of Alloy 718 typically is limited to applications below this temperature.
In order to form the desired precipitation-hardened microstructure, the nickel-base alloys must be subjected to a heat treatment or precipitation hardening process. The precipitation hardening process for a nickel-base alloy generally involves solution treating the alloy by heating the alloy at a temperature sufficient to dissolve substantially all of the γ′-phase and γ″-phase precipitates that exist in the alloy (i.e., a temperature near, at or above the solvus temperature of the precipitates), cooling the alloy from the solution treating temperature, and subsequently aging the alloy in one or more aging steps. Aging is conducted at temperatures below the solvus temperature of the gamma precipitates in order to permit the desired precipitates to develop in a controlled manner.
The development of the desired microstructure in the nickel-base alloy depends upon both the alloy composition and precipitation hardening process (i.e., the solution treating and aging processes) employed. For example, a typical precipitation hardening procedure for Alloy 718 for high temperature service involves solution treating the alloy at a temperature of 1750° F. for 1 to 2 hours, air cooling the alloy, followed by aging the alloy in a two-step aging process. The first aging step involves heating the alloy at a first aging temperature of 1325° F. for 8 hours, cooling the alloy at about 50 to 100° F. per hour to a second aging temperature of 1150° F., and aging the alloy at the second aging temperature for 8 hours. Thereafter, the alloy is air cooled to room temperature. The precipitation-hardened microstructure that results after the above-described heat treatment is comprised of discrete γ′ and γ″-phase precipitates, but is predominantly strengthened by the γ″-phase precipitates with minor amounts of the γ′-phase precipitates playing a secondary strengthening role.
Due to the foregoing limitations, many attempts have been made to improve upon Alloy 718. For example, modified Alloy 718 compositions that have controlled aluminum, titanium, and niobium alloying additions have been developed in order to improve the high temperature stability of the mechanical properties of the alloy. In particular, these alloys were developed in order to promote the development of a “compact morphology” microstructure during the precipitation hardening process. The compact morphology microstructure consists of large, cubic γ′-phase precipitates with γ″-phase precipitates being formed on the faces of the cubic γ′-phase precipitates. In other words, the γ″-phase forms a shell around the γ′-phase precipitates.
In addition to modified chemistry, a specialized heat treatment or precipitation hardening process is necessary to achieve the compact morphology microstructure, instead of the discrete γ′-phase and γ″-phase precipitate hardened microstructure previously discussed. One example of a specialized heat treatment that is useful in developing the compact morphology microstructure involves solution treating the alloy at a temperature around 1800° F., air cooling the alloy, and subsequently aging the alloy at a first aging temperature of approximately 1562° F. for about a half an hour, in order to precipitate coarse γ′-phase precipitates. After aging at the first aging temperature, the alloy is rapidly cooled to a second aging temperature by air cooling, and held at the second aging temperature, which is around 1200° F., for about 16 hours in order to form the γ″-phase shell. Thereafter, the alloy is air cooled to room temperature. As previously discussed, after this precipitation hardening process, the alloy will have the compact morphology microstructure described above and will have improved high temperature stability. However, the tensile strength of alloys having the compact morphology microstructure is generally significantly lower than for standard Alloy 718.
Many γ′-phase strengthened nickel-base alloys exist, for example, Waspaloy® nickel alloy, which is commercially available from Allvac of Monroe, N.C. However, because Waspaloy® nickel alloy contains increased levels of alloying additions as compared to Alloy 718, such as nickel, cobalt, and molybdenum, this alloy tends to be more expensive than Alloy 718. Further, because of the relatively fast precipitation kinetics of the γ′-phase precipitates as compared to the γ″-phase precipitates, the hot workability and weldability of this alloy is generally considered to be inferior to Alloy 718.
Accordingly, it would be desirable to develop an affordable, precipitation-hardened 718-type nickel-base alloy having a microstructure that is predominantly strengthened by the more thermally stable γ′-phase precipitates, that possesses thermally stable mechanical properties at temperatures greater than 1200° F., and that has comparable hot-workability and weldability to γ″-phase strengthened alloys. Further, it is desirable to develop methods of heat treating nickel-base alloys to develop a microstructure that is predominanty strengthened by thermally stable γ′-phase precipitates and that can provide nickel-base alloys with thermally stable mechanical properties and comparable hot-workability and weldability to γ″-phase strengthened alloys.